Now stepping into Garang's shoes, the commander of the Sudan People's Liberation Army is taking up a political role, challenged with making unity with the north attractive to southerners - even as he has called for secession.
Kiir, 54, is to be inaugurated on Thursday as Sudan's first vice president and president of the autonomous government of the south.
"I know him as somebody who thinks before he takes a decision, but when he takes a decision, he sticks with it," said Jan Pronk, the UN representative in Sudan.
"I know him as somebody who has the respect of all the commanders (in the SPLA) and who has respect also from the people in Khartoum because he is a strong military commander."
Fighter to politician
He is known for having a cool head and being able to resolve disputes.
"The man is no slouch intellectually, and he is a leader," said Roger Winter, the US special representative to Sudan.
"He's his own man, a successful man, a well-liked man in the movement, he's got a broad following, he's got a different set of experiences. ... In spite of the fact that he's a military man, he's also got a reputation for being collegial in the way he does business. We all know that wasn't always Dr. John's (Garang's) trait."
Kiir is described by observers as
a natural leader
In the rebel movement that was known for various splits since it was organized in 1983, Kiir stands out as one who never challenged Garang - declaring himself a fighter, not a politician.
Kiir, a member of the Dinka tribe, southern Sudan's largest, joined the separatist Anyanya movement as a teen in the 1960s. When that rebellion ended with a peace deal in 1972, he joined the Sudanese army and rose to the rank of captain.
But in 1983, he joined with Garang in deserting from the army and forming the Sudan People's Liberation Movement/Army, a group that fought for autonomy for the mainly animist and Christian south from the Islamic-dominated northern government.
Kiir had more military experience than Garang, who had also been in the Anyanya but took time out for studies in the United States, and was, therefore, heavily relied upon in the military conflict. Many of the biggest successes in battle were attributed to his leadership in later years, and in 1999 he was made SPLA chief of staff.
Kiir was a key player in early negotiations for a peace deal, leading the southern team to Machakos, Kenya, in 2002 and signing a protocol under which the south was granted the right to hold a referendum on self-determination six years after the signing of the peace deal - which was not agreed upon until January.
Kiir was chief of staff of the
Sudan People's Liberation Army
The new leader was absent from later stages of the negotiations and the agreement under which the south was granted rights to its natural wealth - oil - and power-sharing in the central government, including reservation of the post of first vice president for the SPLM leader.
Shortly after Garang took office in that role, he named Kiir, his longtime deputy, as vice president of the government of southern Sudan.
In the days since Garang's death in a helicopter crash on 30 July, Kiir has promised to continue the late leader's vision of peace for Sudan through implementation of the comprehensive peace agreement. Garang had always said he wanted a unified Sudan, with more autonomy for the south.
"The man is no slouch intellectually, and he is a leader"
Roger Winter, US special representative to Sudan
Kiir, however, was known to favour a separate state in southern Sudan, an ideology that makes him popular in the south.
A recent USAID survey said about 96% of southerners want to secede.
The peace agreement with its power-sharing measures is supposed to help make unity more attractive to southerners before they hold their referendum on secession in 2011.
Kiir has always been popular in the south. As recently as last December, there was a near split within the SPLM when it was rumoured Garang might remove Kiir as chief of staff.
Path to statesman
Ghazi Salah al-Din Atabani, a former presidential adviser in the peace talks and now head of a think tank, said Kiir's calm temperament would help him despite his lack of political experience.
"Those dealing with him are always at ease, more than they used to be in the presence of Garang," Atabani told Alwan newspaper.
"Therefore, he is more capable of handling a political action with wisdom and would be able to unify the southerners."
But Atabani said that Kiir would also have to devote time to northern issues and cultivating international contacts, to become a politician, lest he face difficulties in the national unity government, which includes President Omar al-Bashir and Vice President Ali Osman Mohammed Taha.
"It takes three to tango in this government, and you can't do it very well with one of them limping," Atabani later said.